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My wife's kindergarten class asked, "What would happen to a tree planted on the moon?" Aside from the obvious that it would die from lack of water/air, what physical effects would happen to a tree exposed to the vacuum of Space?

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I'm not sure, but I'd guess that the main thing would be that the water in the cells would evaporate, and what's left behind would shrivel as a result. –  Ted Bunn Apr 1 '11 at 13:16
    
If you managed to feed it by some nutrients, water, light, and CO2, the weaker gravity would make the tree grow differently. It would probably be less straight, unable to find out where is "up" so easily - because of the weaker gravity field. On the other hand, the tree could ultimately be able to become much longer and larger. Or maybe I am wrong? ;-) –  Luboš Motl Apr 1 '11 at 13:24
    
Yes,in a greenhouse on the moon I'd think that tree would grow much larger- Weren't some experiments in ISS with smaller plants? –  Georg Apr 1 '11 at 15:51
    
Plants are constantly absorbing water at the roots and letting it evaporate from the leaves, which propels the flow of water and nutrients up the trunk. In a vacuum this will either dry out the plant or kill it by evaporative cooling. –  endolith Nov 26 '12 at 22:52
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up vote 2 down vote accepted

This is a question for a biologist.

To start with, I guess it would freeze dry, by the water evaporating due to the lack of atmosphere, as Ted observed. If you have forgotten an unwrapped piece of food in the freezer you would know what it means.

Then also the rest of the molecules which are made up mostly of Carbon and Hydrogen would slowly sublimate. If in the sun, much faster.

Now if one assumes a bubble of atmosphere and appropriate water and fertilizer, and the question concerns if the moon soil could support a plant, the answer is , yes, if earth soil microorganisms are introduced into the moon soil where the plant is planted. The technique exists used for improving poor soils. (I recently learned about it from discussions about planting a sour cherry tree in poor soil.)

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